1. Field of the Invention
The present invention relates generally to control rod drives for nuclear reactors and particularly to mechanisms for retarding an undesired ejection of the control rod from the nuclear reactor.
2. Description of the Prior Art
Control rod drives for nuclear reactors comprise sealed pressure vessels which are sealably mounted to the nuclear reactor to connect the control rods of the reactor and to allow the controlled positioning of the rods within the reactor. As such, the housing containing the drive mechanism is maintained at the same pressure as the nuclear reactor. Should the housing of the control rod drive rupture, the leadscrew of the drive mechanism along with the control rod connected thereto could be ejected from the housing by the pressure difference created between the housing and the ambient.
Known prior art control rod drives provide relatively slow movement of the control rods during normal control rod operation with the drive engaging the control rod to prevent the control rod from being ejected. Certain situations are encountered where it is desirable to "scram" or speedily insert the control rod into the nuclear reactor to shut down the reactor. Such a "scram" is usually provided by disengaging the driving element from the control rod and allowing the control rod to drop into the reactor. Thus, during a "scram" condition the control rod could be easily ejected if a rupture were to occur in the housing. A similar condition exists when the reactor is shut down or anytime the drive is disengaged from the control rod. Even when engagement of the control rod with the drive is provided, should a rupture occur which would subject the control rod to the full pressure of the reactor, the engagement between the leadscrew and the drive could be broken or stripped and ejection may still occur. It is therefore highly desirable to retard the ejection of the control rod as much as possible to dissipate the forces ejecting the control rod and prevent catastrophic ejection.